b. Beijing Advanced Innovation Center for Big Data and Brain Computing, Beihang University, Beijing 100191, China;
c. School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, China;
d. Beijing Advanced Sciences and Innovation center, Chinese Academy of Sciences, Beijing 101407, China
Volatile organic compounds (VOCs), such as aromatic, aliphatic hydrocarbons, and chlorinated hydrocarbons, are the major pollutants in indoor air and have serious environmental and health impacts. Photocatalytic oxidation (PCO) is one of the most active routes to eliminate most VOCs because it is cost-effective and environmentally friendly [1, 2]. TiO2 is currently widely used in the field of degrading VOCs due to its low price, sensitive photoactivation, and high chemical stability [3, 4]. In the past decades, extensive efforts have been devoted to improving the performance of TiO2-based photocatalysts using various methods, including surface modification [5, 6], metal ions (Ru, Ag, Au, and Pt) doping [7-9], non-metal ions (F, N, C, and S) doping [10-12], and coupling with a semiconductor such as CdS or SnO2 [13, 14]. Mechanistic studies of VOCs degradation have been mainly focused on the adsorption sites, reaction mechanism, and kinetics [15, 16].
The adsorption of reactant on the surface of photocatalyst is the first crucial step for photo-degradation. Maira and Fresno et al. [17, 18] reported that the toluene was adsorbed on the TiO2 via a
Surface modification of TiO2 is an effective approach to improve the photocatalytic performance [23-26]. For example, surface fluorination of TiO2 not only traps the photegenerated electrons due to the high electronegativity of fluorine, but also favors the transformation of holes and promotes the production of surface hydroxyl radicals . The TiO2 modification via sulfation can promote the formation of surface sulfates, producing more hydroxyl groups than that of the pure TiO2 and thus enhancing its degradation activity. We have investigated systematically the modification of TiO2 with NH3 and H2S for the toluene degradation. The adsorption mechanism and relationship of surface structure-performance were established .
Facet engineering of semiconductor photocatalysts is a promising route to improve the photocatalytic performance . Generally, the crystal facets with more undercoordinated atoms are more reactive in photocatalytic reactions. For example, the (001) facet of anatase TiO2 contains 100% 5-fold coordinated Ti atoms in comparison to 50% 5c-Ti on the (101) facet, thus exhibiting higher photocatalytic performance . Furthermore, for TiO2, the photogenerated holes and electron can selectively migrate toward to (001) facets and (101) facets, forming an oxidation site and a suitable redox site, respectively . In previous work , we found that the (001) facets of TiO2 exhibit better toluene degradation activity than that of the (101) facets. The reason is that the (001) facets of TiO2 provide a higher adsorption ability for toluene and molecular water.
Ozone has a high redox potential (
TiO2 with dominant (001) facet was prepared by a hydrothermal method [35, 36]. In a typical procedure, 9 mL of HF solution (40 wt%) as a shape-directing agent was first added into 30 mL of Ti(OC
The ozone modification of the as-synthesized TiO2 was performed on an ozone generator (PCE-22-LD, Hefei KeJing materials technology Co., Ltd.) equipped with a 200 W UV lamp. First, 0.01 g TiO2 was dispersed in a moderate amount of deionized water, which was then uniformly coated on a glass substrate. After drying at 70 ℃ for 10 min, the sample was placed in ozone generator and radiated under the UV light. The obtained sample was denoted as O3/TiO2.B. Characterization of photocatalysts
The powder X-ray diffraction (XRD) were performed on an instrument (TTRIII, Rigaku) with Cu K
The catalytic performance for toluene degradation was tested on a home-made system, which consisted of a gas supply system, a 400 mL Pyrex glass photoreactor, and a gas chromatograph analytical system, as shown in FIG. S1 in supplementary materials. The light source was a 300 W Xe-arc lamp equipped with an IR-cutoff filter for eliminating the thermal effect (PLS-SXE 300UV, Beijing Perfectlight Technology Co., Ltd.).
In each experiment, the glass substrate coated with 0.01 g photocatalyst was placed into the photoreactor and 0.2 μL toluene was introduced into the photoreactor by injection. Before each reaction, the catalyst was first placed in the darkness for 0.5 h to achieve an adsorption/desorption equilibrium, and then illuminated by the Xe-arc lamp.
During the reaction, the concentrations of gaseous toluene were measured at an interval of 15 min with a GC (GC1690, Hangzhou Kexiao, China) equipped with a KX-112 column and a flame ionization detector (FID).D. In situ DRIFTS test
The In situ DRIFTS experiments were tested in Shanghai Synchrotron Radiation Facility and the National Synchrotron Radiation Laboratory at University of Science and Technology of China.
A Nicolet 8700 FT-IR spectrometer equipped with a MCT detector was employed. The schematic diagram of the set-up is shown in FIG. S1 in supplementary materials). The reaction system consists of a DRIFTS accessory and a reaction cell (Harrick Scientific). The dome covering the sample cup has three windows: two ZnSe IR windows and one quartz window for introducing the irradiation light. The light source is monochromatic ultraviolet light (365 nm) with a power of 0.72 W (Honle, GBACS70-24-C14). In the experiment, the vapor of toluene was introduced into the reaction cell by dry air bubbling through the toluene solution.Ⅲ. RESULTS AND DISCUSSION
FIG. 1 (a) and (b) show the SEM and TEM images of the as-prepared TiO2, which indicate that the TiO2 particles have a regular nanosheet-like shape with a lateral size of about 100 nm
The XRD patters of the pristine TiO2 and ozone modified-TiO2 are shown in FIG. 2. The diffraction patterns of both samples are indexed to the pure anatase TiO2 (JCPDS No.21-1272). It is worth noting that the (004) peak is widened while (200) peak is sharp, indicating that the TiO2 particles have a small grain size along the  direction and a large size along the  direction. The results further suggest that the as-prepared TiO2 has a nanosheet-like structure, which is consistent with the SEM/TEM results. No obvious differences are observed between the XRD patterns of the pure TiO2 and ozone modified sample, indicating that the crystalline phase of TiO2 has not been changed by the surface modification of ozone.
FIG. S2 of supplementary materials shows the LRS of the pure TiO2 and ozone modified-TiO2. There are four peaks at the position of 141 cm-1 (E
The optical absorbances of TiO2 before and after ozonation modification were characterized by UV-Vis DRS. As shown in FIG. S3 (supplementary materials), both of the two samples exhibit a sharp absorbance in the range of 300-400 nm-1, and no obvious difference in their cut-off wavelengths is observed. The estimated band gap for the pure TiO2 and ozone-modified TiO2 are 3.0 eV and 3.02 eV respectively, which means that the band gaps of TiO2 were not changed by ozonation treatment.
XPS spectrum was used to investigate the surface chemical environments of catalysts. Only Ti and O features were detected. No fluoride species were observed in the XPS spectra, indicating that fluoride species have been removed after the synthetic process. As shown in FIG. 3, two characteristic binding energies of Ti 2p
To investigate the generation of active radicals in the catalysts during the photocatalytic reaction, the electron paramagnetic resonance (EPR) spin trapping technique was applied using DMPO as spin trap and the results are shown in FIG. 4. No EPR signal was detected when experiment was carried out in the dark or only DMPO was irradiated. In FIG. 4(a), a characteristic quadruplet with a signal-to-intensity ratio of 1:2:2:1 was observed, which demonstrated that both the samples produced OH
The (001) facets of anatase TiO2 contain a large amount of unsaturated coordinated Ti5c atoms, which have a strong Lewis acidity . After ozone adsorbing on the strong Lewis Ti sites, it will be further distorted and unstable until a surface oxygen atom (Ti-O) and a free oxygen molecule are separated . In order to understand the reaction mechanism, DFT was employed to investigate the reaction pathway of ozone adsorption and dissociation on the surface of TiO2 (001) (see details in supplementary materials) [40, 41]. The energy profile of the process is shown in FIG. 5. Ozone molecule adsorbs on the surface of (001) facets through an O-end bonding with Ti5c, the weakened O-O bond breaks to form Ti5c-O on the surface. The adjacent Ti5c-O combines to form more stable Ti5c-O-O-Ti5c. The energy barrier of this step is only 1.11 eV and the process is exothermic by 0.77 eV. Subsequently, the environmental water absorbs on Ti5c-O through H-O-H end bonding with Ti5c-O; then the weakened O-H breaks and forms isolated Ti5c-OH and dissociated hydroxyl radicals. The process is also exothermic. Based on the above analysis, the ozone-modified (001) dominated TiO2 can promote the formation of isolated Ti5c-OH and hydroxyl radicals, which is consistent with experimental observation by XPS, EPR and following In situ DRIFTS.
The photocatalytic activity of the two photocatalysts for toluene elimination with the condition of room humidity 40% and initial toluene concentration of 100 ppmv were tested and the results are shown in FIG. 6. The photocatalyst treated with ozone exhibited a faster degradation rate for toluene than the pristine TiO2 without ozone modification, as shown in FIG. 6(a). The degradation rate of toluene increased with an increase in the ozone treatment time. The Langmuir-Hinshelwood model was employed to fit the degradation process using a reaction rate equation:
In order to study the adsorption and degradation process of toluene on the ozone-modified sample, the In situ DRIFTS was performed. FIG. 7 shows the DRIFTS of the fresh TiO2 and O3-modified TiO2 samples. The narrow peak at 3693 cm
The In situ DRIFTS for the adsorption of toluene on the two photocatalysts are shown in FIG. 8. With an increase of the adsorption time of toluene, the intensity of peak assigned to the isolated hydroxyl group (3693 cm
After the adsorption toluene reached equilibrium, the samples were illuminated by UV lamp and the In situ DRIFTS were recorded. As shown in FIG. 10, with increasing the illumination time, the intensity of broad peak at 3580 cm-1 gradually decreased, and several new peaks (1638, 1603, 1581, 1498, 1456, 1416, 1320 and 1308 cm-1) appeared in the range of 1800-1300 cm-1. The absorption bands at 1638, 1603, and 1581 cm-1 can be assigned to the benzaldehyde, and the bands of 1456, 1416 cm-1 are related to the benzoic acid . The band of 1498 cm-1 is assigned to the in-plane skeletal vibration mode of aromatic ring , while the narrow bands at 1320 and 1308 cm-1 correspond to the in-plane rocking vibration of CH3 . The benzaldehyde and benzoic acid are the key intermediates for the toluene degradation. The toluene molecules were first oxidized to the intermediate benzaldehyde, and then further converted to benzoic acid under illumination, which is consistent with our previous study [19, 29]. Compared with pure TiO2, it is noted that the ozone-modified TiO2 shows a higher peak intensity ratio of benzaldehyde (1638, 1603, 1581 cm-1) adsorption on the ozone-modified TiO2, the results demonstrate that ozone-treated TiO2 exhibited a faster rate of photocatalytic degradation of toluene than the pristine sample.
Reactive oxygen species (ROS), typically including hydroxyl radical (OH
In summary, we synthesized the (001) facets dominated TiO2 nanosheets and investigated in detail the effect of ozone modification on the performance of the photocatalyst for photocatalytic degradation of toluene. The ozone-modified TiO2 showed better performance for toluene degradation than the pure TiO2. The detailed characterizations indicate that the ozone molecules were adsorbed on the abundant unsaturated coordinated 5c-Ti sites on (001) facets. The formed Ti-O bonds via ozone dissociation reacted with H2O to produce a large amount of isolated Ti5c-OH on the surface. The Ti5c-OH act as the adsorption sites for toluene, thus significantly improving the adsorption ability of the TiO2 for toluene. The enhanced performance of the ozone-modified TiO2 is owing to its high adsorption ability for toluene and the abundant surface hydroxyl groups, which can produce very reactive OH
Supplementary materials: Schematic diagram of In situ DRIFTS setup for toluene adsorption, photoreactor, details of IR cell, Raman spectrum, UV-vis absorption spectrum and computational details.Ⅴ. acknowledgements
This work was supported by the National Natural Science Foundation of China (U1632273, 21673214, U1732272, U1832165).
A. Fujishima, and K. Honda, Nature 238, 37(1972). DOI:10.1038/238037a0
T. Guo, Z. Bai, C. Wu, and T. Zhu, Appl. Catal. B 79, 171(2008). DOI:10.1016/j.apcatb.2007.09.033
B. Oregan, and M. Gratzel, Nature 353, 737(1991). DOI:10.1038/353737a0
A. Fujishima, X. Zhang, and D. A. Tryk, Surf. Sci. Rep. 63, 515(2008). DOI:10.1016/j.surfrep.2008.10.001
M. Sabzi, S. M. Mirabedini, J. Zohuriaan-Mehr, and M. Atai, Prog. Org. Coat. 65, 222(2009). DOI:10.1016/j.porgcoat.2008.11.006
H. Park, Y. Park, W. Kim, and W. Choi, J. Photochem. Photobiol. C 15, 1(2013). DOI:10.1016/j.jphotochemrev.2012.10.001
M. V. Dozzi, A. Saccomanni, and E. Selli, J. Hazard. Mater. 211, 188(2012).
T. Chen, G. P. Wu, Z. C. Feng, J. Y. Shi, G. J. Ma, P. L. Ying, and C. Li, Chin. J. Chem. Phys. 20, 483(2007).
Y. Ma, X. L. Wang, Y. S. Jia, X. B. Chen, H. X. Han, and C. Li, Chem. Rev. 114, 9987(2014). DOI:10.1021/cr500008u
T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, and Y. Taga, Jpn. J. Appl. Phys. Part 2. 40, L561(2001). DOI:10.1143/JJAP.40.L561
J. Yuan, M. Chen, J. Shi, and W. Shangguan, Int. J. Hydrogen Energy 31, 1326(2006). DOI:10.1016/j.ijhydene.2005.11.016
X. Wu, S. Yin, Q. Dong, C. Guo, H. Li, T. Kimura, and T. Sato, Appl. Catal. B 142, 450(2013).
Y. Bessekhouad, D. Robert, and J. Weber, J. Photochem. Photobiol. A 163, 569(2004). DOI:10.1016/j.jphotochem.2004.02.006
L. Y. Shi, C. Z. Li, H. C. Gu, and D. Y. Fang, Chin J. Chem. Phys. 13, 336(2000).
W. W. Lai, M. Hsu, and A. Y. Lin, Water Res. 112, 157(2017). DOI:10.1016/j.watres.2017.01.040
J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, and D. W. Bahnemann, Chem. Rev. 114, 9919(2014). DOI:10.1021/cr5001892
A. J. Maira, K. L. Yeung, J. Soria, J. M. Coronado, C. Belver, and C. Y. Lee, Appl. Catal. B 29, 327(2001). DOI:10.1016/S0926-3373(00)00211-3
F. Fresno, M. D. Hernandez-Alonso, D. Tudela, J. M. Coronado, and J. Soria, Appl. Catal. B 84, 598(2008). DOI:10.1016/j.apcatb.2008.05.015
F. Zhang, M. J. Wang, X. D. Zhu, B. Hong, W. D. Wang, Z. M. Qi, W. Xie, J. J. Ding, J. Bao, S. Sun, and C. Gao, Appl. Catal. B 170, 215(2015).
K. L. Yeung, S. T. Yau, A. J. Maira, J. M. Coronado, J. Soria, and P. L. Yue, J. Catal. 219, 107(2003). DOI:10.1016/S0021-9517(03)00187-8
E. Farfan-Arribas, and R. J. Madix, J. Phys. Chem. B 107, 3225(2003). DOI:10.1021/jp022344c
K. I. Hadjiivanov, and D. G. Klissurski, Chem. Soc. Rev. 25, 61(1996). DOI:10.1039/cs9962500061
H. Zhang, M Cai J., Y. T. Wang, M. Q. Wu, M. Meng, Y. Tian, X. G. Li, J. Zhang, L. R. Zheng, Z. Jiang, and J. L. Gong, Appl. Catal. B 220, 126(2018). DOI:10.1016/j.apcatb.2017.08.046
M Cai J., M. Q. Wu, Y. T. Wang, H. Zhang, M. Meng, Y. Tian, X. G. Li, J. Zhang, L. R. Zheng, and J. L. Gong, Chem. 2, 877(2017). DOI:10.1016/j.chempr.2017.05.006
J. M Cai, Y. T. Wang, Y. M. Zhu, M. Q. Wu, H. Zhang, X. G. Li, Z. Jiang, and M. Meng, ACS Appl. Mater. Inter. 7, 24987(2015). DOI:10.1021/acsami.5b07318
Y. T. Wang, J. M Cai, M. Q. Wu, H. Zhang, M. Meng, Y. Tian, T. Ding, J. L. Gong, Z. Jiang, and X. G. Li, Appl. Mater. Inter. 8, 23006(2016). DOI:10.1021/acsami.6b05777
L. Y. Zong, G.D. Zhang, H. J. Zhao, J. Y. Zhang, and Z. C. Tang, Chem. Eng. J. 354, 295(2018). DOI:10.1016/j.cej.2018.07.199
N. Roy, Y. Sohn, and D. Pradhan, ACS Nano. 7, 2532(2013). DOI:10.1021/nn305877v
M. J. Wang, F. Zhang, X.D. Zhu, Z. M. Qi, B. Hong, J. J. Ding, J. Bao, S. Sun, and C. Gao, Langmuir 31, 1730(2015). DOI:10.1021/la5047595
F. J. Beltran, A. Aguinaco, J. F. Garcia-Araya, and A. L. Oropesa, Water Res. 42, 3799(2008). DOI:10.1016/j.watres.2008.07.019
P. Fu, P. Zhang, and J. Li, Appl. Catal. B 105, 220(2011). DOI:10.1016/j.apcatb.2011.04.021
Z. Jiang, M. X. Chen, J. W. Shi, J. Yuan, and W. F. Shangguan, Catal. Surv. Asia 19, 1(2015). DOI:10.1007/s10563-014-9177-8
L. Sanchez, J. Peral, and X. Domenech, Appl. Catal. B 19, 59(1998). DOI:10.1016/S0926-3373(98)00058-7
H. C. Wang, W. Q. Guo, Z. Jiang, Yang Ruoou, Z. Jiang, Y. Pan, and W. F. Shangguan, J. Catal. 361, 370(2018). DOI:10.1016/j.jcat.2018.02.023
C. Liu, X. Han, S. Xie, Q. Kuang, X. Wang, M. Jin, Z. Xie, and L. Zheng, Chem-Asian J. 8, 282(2013). DOI:10.1002/asia.201200886
Q. Xiang, K. Lv, and J. Yu, Appl. Catal. B 96, 557(2010). DOI:10.1016/j.apcatb.2010.03.020
W. Su, J. Zhang, Z. Feng, T. Chen, P. Ying, and C. Li, J. Phys. Chem. C 112, 7710(2008). DOI:10.1021/jp7118422
F. Tian, Y. Zhang, J. Zhang, and C. Pan, J. Phys. Chem. C 116, 7515(2012). DOI:10.1021/jp301256h
K. M. Bulanin, J. C. Lavalley, and A. A. Tsyganenko, J. Phys. Chem. 99, 10294(1995). DOI:10.1021/j100025a034
P. Zhou, X. Zhu, J. Yu, and W. Xiao, ACS Appl. Mater. Inter. 5, 8165(2013). DOI:10.1021/am402246b
G. Zhou, Y. Shan, Y. Y. Hu, X. Y. Xu, L. Y. Long, J. L. Zhang, J. Dai, J. H. Guo, J. C. Shen, S. Li, L. Z. Liu, and X. L. Wu, Nat. Commun. 9, 3366(2018). DOI:10.1038/s41467-018-05590-x
C. Morterra, J. Chem. Soc., Faraday Trans. Ⅰ. 84, 1617(1988). DOI:10.1039/f19888401617
G. Cerrato, L. Marchese, and C. Morterra, Appl. Surf. Sci. 70(93), 200(1993).
M. Nagao, and Y. Suda, Langmuir 5, 42(1989). DOI:10.1021/la00085a009
L. X. Cao, Z. Gao, and S. L. Suib, J. Catal. 220, U269(2000).
H. Hatakeyama, C. Nagasaki, and T. Yurugi, Carbohydr. Res. 48, 149(1976). DOI:10.1016/S0008-6215(00)83211-5
Y. Nosaka, and A. Y. Nosaka, Chem. Rev. 117, 11302(2017). DOI:10.1021/acs.chemrev.7b00161
b. 北京航空航天大学北京大数据与脑计算高级创新中心，北京 100191;
c. 安徽大学化学化工学院，合肥 230601;
d. 中国科学院北京高等科学与创新中心，北京 101407